I am considering creating a small above ground pond from a vintage bathtub that rests against my house. The tub could hold up to approximately 60 gallons and I think it would make an interesting summer home for my two commons.

However, I am worried about pollutants spoiling the water. I wouldn't mind doing water changes, as needed. Of course the pH would need to be looked after, especially due to the often acidic nature of rain in the city.

What sort of experiences and problems have you encountered in a small pond located in a fairly polluted city?